ABSTRACT Alveologenesis occurs during the last stage of lung development and is responsible for subdividing the terminal airways of the lungs into alveoli, structures that are critical for efficient gas exchange in the lung. Defects in this process lead to the formation of simplified alveoli that are a hallmark of bronchopulmonary dysplasia (BPD), a chronic lung disease that presents in premature infants treated with mechanical ventilation or oxygen therapy. Although the molecular mechanisms that regulate alveolar development during postnatal stages are unknown, mesodermally-derived fibroblasts in the lung mesenchyme have been shown to be critical drivers of alveologenesis. Published work from our laboratory has demonstrated that all three Hox5 genes (HoxA5, HoxB5 and HoxC5) are exclusively expressed in the lung mesenchyme, and loss of all three Hox5 genes leads to severe developmental lung defects and perinatal death. Four-allele, compound Hox5 mutant mice (Hox5 AabbCc) are born in Mendelian ratios and their lungs are histologically normal at birth, however, they develop alveolar simplification at postnatal stages. Consistent with a direct role for Hox5 genes in alveologenesis, the expression levels of all three Hox5 genes peak during the postnatal stages when alveologenesis is at its peak. Our laboratory has recently generated a conditional allele for Hoxa5, allowing us to bypass the neonatal lethality and assess the post-embryonic functions of this group of regulators. Conditional deletion of Hoxa5 in the lung mesenchyme beginning at birth results in alveolar simplification postnatally. The addition of null alleles for Hoxb5 and Hoxc5 exacerbate this defect. Hox5 conditional mutant lungs exhibit abnormal myofibroblast distribution, shape and impaired function, and the elastin network required for proper alveologenesis fails to form. Unbiased RNAseq analyses reveal gene expression changes in categories associated with cell adhesion and extracellular matrix. Immunofluorescence and western blot analyses demonstrate that both the basement membrane and extracellular matrix components are expressed normally in Hox5 conditional mutants. However, mutant fibroblasts exhibit significant adhesion defects in culture, and preliminary data show loss of Integrin?5 expression in fibroblasts derived from Hox5 conditional mutants. Collectively, our data indicate that Hox5 genes regulate the proper differentiation and function of mesenchymal fibroblasts and control lung matrix formation critical for alveologenesis. Using genetics, I plan to elucidate the cellular and molecular mechanisms of Hox5 regulation of lung mesenchyme in alveologenesis.